1. Field of the Invention
The present invention relates to a color image compressing method for compressing color images read by a scanner or the like and a device thereof.
2. Description of the Related Art
JPEG (joint photographic coding experts group) coding is conventionally known as an image compressing method. JPEG coding divides an image into a plurality of blocks, performs discrete cosine transform (DCT) and quantizes/encodes the obtained values.
JPEG coding is suitable for compressing images, such as a photograph and the like. However, since image information is compressed by coarsely quantizing its RF element, the edge of a character that has a large change in brightness and color and the like becomes unclear, which is a problem.
In order to solve such a problem, Patent reference 1 discloses a technology for transmitting/receiving images superior in reproducing black edges by extracting the black edge of a color image, encoding the location of the black edge, transmitting it together with a color image encoded by a JPEG method and compensating for the pixel of the black edge, based on the black edge information decoded on the receiving side.
Patent reference 2 discloses an area determining unit dividing an image into character area data and photograph area data, and extracting both data, a photograph image encoding unit encoding photograph area data, and a monochrome image encoding unit encoding digitized character area data.
If a manuscript including characters and the like is read by a scanner, the outline of a character area takes neutral tints and no sharp edges exist. Therefore, no clear area determination can be conducted, which is another problem. In order to solve such a problem, Patent reference 3 discloses an outline correcting unit compensating for a pixel recognized as neutral tints, based on the color difference between its adjacent pixels.
However, since in the method of Patent reference 1, data indicating the location of a black edge must be transmitted separately from color image data, it is difficult to improve a data compression ratio.
In the compression method of Patent reference 2, although a photograph area and a character area are distinguished, this distinction depends on the fact that at the edge of a character area, a pixel with a high density level and a pixel with a low density level adjoin each other.
In the method of Patent reference 3, area determination accuracy is simply improved by replacing the value of a pixel with neutral tints in the outline of an area, with the value of its adjacent pixel whose color difference is a minimum.
All the above-mentioned conventional compression methods realize compression by reducing the redundancy of image data after being read by a scanner and the like, and can not reduce redundancy caused when the scanner and the like reads a manuscript, by extracting the features of the ruled lines, characters or the like of the original manuscript. Therefore, a color image including a ruled line, a character and the like cannot be compressed at a high compression ratio and also the outline of a character and the like cannot be clearly displayed.
Patent Reference 1:
Japanese Patent Application Laid-open No. 8-279913
Patent Reference 2:
Japanese Patent Application Laid-open No. 8-298589
Patent Reference 3:
Japanese Patent Application Laid-open No. 2001-61062
Patent Reference 4:
                Japanese Patent Application Laid-open No. 8-16793        